The world’s biggest fish are female © Erick Higuera / Getty

The world’s biggest fish are female

Female whale sharks rule the oceans, reaching average lengths of 14 metres.

When it comes to whale sharks, the female of the species is more lengthy than the male, scientists have discovered. This makes female whale sharks the world’s largest living fish, typically five metres longer than their male counterparts.

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Whale sharks – slow-moving filter feeders that can be found throughout the world’s tropical oceans – have previously been found to grow up to 18m long – about the length of a bendy bus.

They’re classed as an endangered species, threatened by fishing and collisions with ships, so researchers want to find out more about their lives in order to work out ways to better protect them.

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In this study, marine scientists visited Australia’s Ningaloo Reef – a whale shark feeding ground – for 11 years between 2009 and 2019, tracking 54 whale sharks as they grew. The researchers could identify each whale shark by the unique ‘fingerprint’ of spots on its body, and they took more than 1,000 measurements in total, using ‘stereo-video’ cameras.

“It’s basically two cameras set up on a frame that you push along when you’re underwater,” said co-author Dr Brett Taylor at the Australian Institute of Marine Science (AIMS). “It works the same way our eyes do, so you can calibrate the two video recordings and get a very accurate measurement of the shark.”

A marine scientist uses a stereo-video camera to measure the length of a whale shark
Using a stereo-video camera to measure a whale shark © Andre Rereuka / AIMS

The researchers found that male whale sharks initially grow more quickly than the females, but plateau at an average length of about eight or nine metres, whereas the females overtake them to reach an average length of 14m.

Lead researcher Dr Mark Meekan said that there are advantages to the females to being so big.

“Only one pregnant whale shark had ever been found, and she had 300 young inside her,” said Meeken, who also works at AIMS. “That’s a remarkable number, most sharks would only have somewhere between two and a dozen. So these giant females are probably getting big because of the need to carry a whole lot of pups.”

Although female whale sharks are huge, the study also found that they grow extremely slowly – only 20 or 30cm a year, which has implications for conservation.

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“If you’re a very slow-growing animal and it takes you 30 years or more to get to maturity, the chances of disaster striking before you get a chance to breed is probably quite high,” said Meekan. “And that’s a real worry for whale sharks.”

Reader Q&A: How do sharks smell blood underwater?

Asked by: Todd Michael Wilson, USA

When you smell something in the air, it’s because scent molecules have dissolved into the wet lining of your nose. Smelling underwater is no different, except that the molecules are already dissolved in the seawater. It’s a myth that sharks can smell a single drop of blood from a mile away.

Sharks actually have roughly the same sensitivity as other fish and can detect smells at between one part per 25 million and one part per 10 billion, depending on the chemical, and the species of shark. At the top end, that’s about one drop of blood in a small swimming pool.

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